Semiconductors form the basis of modern electronics. Possessing physical properties that can be selectively modified and controlled between conduction and insulation, semiconductors are essential in most modern electrical devices (e.g., computers, cellular phones, photovoltaic cells, etc.). Group IV semiconductors generally refer to those first four elements in the fourth column of the periodic table (e.g., carbon, silicon, germanium and tin).
The ability to deposit semiconductor materials using non-traditional semiconductor technologies such as printing may offer a way to simplify and hence reduce the cost of many modern electrical devices (e.g., computers, cellular phones, photovoltaic cells, etc.). Like pigment in paint, these semiconductor materials are generally formed as microscopic particles, such as nanoparticles, and temporarily suspended in a colloidal dispersion that may be later deposited on a substrate.
Nanoparticles are generally microscopic particles with at least one dimension less than 100 nm. In comparison to a bulk material (>100 nm) which tends to have constant physical properties regardless of its size (e.g., melting temperature, boiling temperature, density, conductivity, etc.), nanoparticles may have physical properties that are size dependent, such as a lower sintering temperature.
A colloidal dispersion is a type of homogenous mixture consisting of two separate phases. A colloidal dispersion (or ink) generally consists of a continuous phase (such as a solvent), and a dispersed phase (generally particles under 1 um in diameter). The continuous phase must be compatible with the surface of the material to be dispersed. For example, carbon black particles (non-polar) tend to be easily dispersed in a hydrocarbon solvent (non-polar), whereas silica particles (polar) tend to be easily dispersed in alcohol (polar).
Polarity generally refers to the dipole-dipole intermolecular forces between the slightly positively charged end of one molecule to the negative end of another or the same molecule. However, semiconductor particles tend to be non-polar, and hence lyophobic (or solvent fearing).
It is often of benefit to functionalize semiconductor surfaces by the addition of capping agents in order to improve compatibility with the media and simplify and/or enable manufacturing processes. In general, a capping agent or ligand is a set of atoms or groups of atoms bound to a “central atom” in a polyatomic molecular entity. The capping agent is selected for some property or function not possessed by the underlying surface to which it may be attached.
Consequently, a common method of dispersing a non-polar particle in a polar solvent is through modification of the particle surface, often with an ionizable (or polar organic) capping agent or ligand. For example, ionizable functional groups, such as carboxyl, amino, sulfonate, etc. or polymeric forms thereof, are often covalently attached to non-polar particles in order to add charge and allow repulsive electrostatic forces aid in the dispersion of the particles in the solvent. Alternatively in the case of apolar solvents, non-ionizable organic groups, highly compatible with the solvent, may be covalently grafted to the particles to aid dispersion and impart stability via solvation forces. Examples of non-ionizable organic groups include different geometry hydrocarbons (e.g., linear, branched and cyclo-alkanes, alkenes, alkynes, cycloalkanes, alkadienes, etc.).
In addition, once dispersed, these particles will tend to stay suspended and avoid agglomeration if the repulsive electrostatic and/or solvation forces are sufficiently higher than the normally attractive Van der Waals forces. If the repulsive barrier to Van der Waals interactions is higher than about 15 kT, then Brownian motion of the particles is too low to cause appreciable agglomeration and the dispersion is considered stable. This balance of energies is the essence of Derjaguin-Landau-Verwey-Overbeek (DLVO) theory used to explain stability of electrostatically-stabilized colloids.
Capping agents can also attach antimicrobial molecules (e.g., polycationic (quaternary ammonium), gentamycin, penicillin, etc.) on a Group IV semiconductor surface in order to protect people from microbial infection. For example, Group IV materials with antimicrobial capping agents may be used for making clothing that can be more safely worn in contaminated environments.
However, in these and other uses, it is generally difficult to selectively attach the capping agent to the Group IV semiconductor surface, since the surface's chemical structure tends to be uniform and homogeneous. Consequently, the capping agent tends to attach to all available surface sites reactive toward it, completely covering the surface. This may be problematic for applications which require a direct access to the Group IV semiconductor surface. For example, an excessive amount of capping agents may inhibit sintering. Sintering is generally a method for making objects from powder by heating the particles below their melting point until they adhere to each other.
In addition, once attached to the Group IV semiconductor surface, capping ligands may be difficult to remove and may consequently interfere with the surface functionality. For example, many capping agents (once deposited and sintered) may act as contaminants which detrimentally affect the electrical characteristics of the semiconductor particle.
In view of the foregoing, there is desired a method of selectively activating hydrogen passivated Group IV semiconductor surfaces.